The invention relates to TV tuners, and in particular, to TV tuners for television systems.
FIG. 1 shows a conventional down-conversion TV tuner, comprising a phase locked loop 11, an input tuning circuit 12, a radio frequency (RF) amplifier 13, an intermediate tuning circuit 14, an oscillator 15, a mixer 16, a band-pass filter 17, an intermediate amplifier 18 and a demodulator 19.
When an output signal of the RF amplifier 13 has a carrier frequency fc, the oscillator 15 has an oscillation frequency fLO, and an intermediate frequency signal output from the mixer 16 has a frequency fIF, according to the formula:fIF=fLO−fc 
Accordingly, oscillation frequency fLO is higher than the carrier frequency fc of the output signal, input to the mixer 16. In this case, the down-conversion TV tuner refers to a superheterodyne receiver, and operation in the mixer 16 is defined as high-side injection. In the down-conversion TV tuner, however, image frequency and the intermediate frequency signal mix together, inducing deterioration of image quality thereof.
FIG. 2 shows a double-conversion TV tuner as disclosed in U.S. Pat. No. 5,737,035. RF signal received by an antenna 202 is transmitted to the input tuning circuit 204, amplified by the low-noise transconductance amplifier (LNTA) 206, and sent to the mixers 208 and 210. Mixers 208 and 210 mix the received signals with oscillation signals generated by the oscillators 250 and 212, outputting an intermediate frequency signal from the mixer 210. An input signal of the mixer 208 has a carrier frequency of 0 to 900 MHz, and an input signal of the mixer 210 has a carrier frequency of 1200 MHz. The oscillator 250 has an oscillation frequency of 1200 to 2100 MHz, and the oscillator 212 has an oscillation frequency of 1180 MHz. In this case, the oscillation frequency of the oscillator 250 is higher than the carrier frequency of the input signal of the mixer 208, and the mixer 208 performs high-side injection. Alternatively, the oscillation frequency of the oscillator 212 is lower than the carrier frequency of the input signal of the mixer 210, so the mixer 210 performs low-side injection.
TV tuners can be designed differently in accordance with application in different systems. Accordingly, conventional TV tuners are categorized as single-system TV tuners for NTSC(M) or PAL(M/N, B/G, D, I) TV systems and multi-system TV tuners for NTSC(M) and PAL(M/N, B/G, D, I), or SECAM(B/G, D/K, I, L/L′) and PAL(M/N, B/G, D, I) TV systems. There is need for a TV tuner applicable in both single-system and multi-system TV systems.
In the TV systems, however, L′ system is significantly different from other TV systems in bandwidth and carrier frequency of the audio signal. Accordingly, cost of a conventional TV tuner applicable to all TV systems is high.
FIG. 3A shows a frequency spectrum diagram of a single-conversion of M/N, B/G, D/K, I and L TV systems. The carrier frequency Sc of the RF audio signal is higher than the carrier frequency Pc of the RF video signal. The RF signals undergo single-conversion such that the carrier frequency Sc′ of the intermediate frequency audio signal is lower than the carrier frequency Pc′ of the intermediate frequency video signal. In FIG. 3B, however, a frequency spectrum diagram of L′ TV system shows that the carrier frequency Sc of the RF audio signal is lower than the carrier frequency Pc of the RF video signal. The RF signals undergo single-conversion such that the carrier frequency Sc′ of the intermediate frequency audio signal is higher than the carrier frequency Pc′ of the intermediate frequency video signal. Thus, in the conventional TV tuner with single-conversion, an additional double band-pass surface acoustic wave (SAW) filter with dual Nyquist slope is required to filter the converted intermediate frequency signal.
Further, in the conventional TV tuner with double-conversion, two mixers perform high-side injection and low-side injection respectively such that frequency spectrum diagrams are similar to those in single-converse. Thus, an additional SAW filter is required.